Fortifying himself strongly in his dwelling and in the quarters of his men, in Tezcoco, he at once applied himself to the task of securing such military positions in the valley and in the neighborhood of the great causeway between the lakes as would command an outlet from the capital by land, and enable him to advance across the waters of Tezcoco without the annoyance of enemies who might sally forth from strongholds on his left flank. On his right, the chain of lakes, extending farther than the eye can reach, furnished the best protection he could desire. Accordingly, he first of all reduced and destroyed the ancient city of Iztapalapan,--a place of fifty thousand inhabitants, distant about six leagues from the town of Tezcoco,--which was built on the narrow isthmus dividing the lake of that name from the waters of Chalco. He next directed his forces against the city of Chalco, lying on the eastern extremity of the lake that bore its name, where his army was received in triumph by the peaceful citizens after the evacuation of the Mexican garrison. Such were the chief of his military and precautionary expeditions, until the arrival of the materials for the boats or brigantines which Martin Lopez, and his four Spanish assistant carpenters, had already put together and tried on the waters of Zahuapan; and which, after a successful experiment, they had taken to pieces again and borne in fragments to Tezcoco. * * * * * Early in the spring of 1521, Cortéz entrusted his garrison at Tezcoco to Sandoval, and, with three hundred and fifty Spaniards, and nearly all his Indian allies, departed on an expedition designed to reconnoitre the capital. He passed from his stronghold northwardly around the head of the lakes north of Tezcoco,--one of which is now called San Cristoval,--and took possession of the insular town of Xaltocan. Passing thence along the western edge of the vale of Anahuac or Mexico, he reached the city of Tacuba, west of the capital, with which so many disastrous recollections were connected on his first sad exit from the imperial city. During this expedition the troops of the conqueror were almost daily engaged in skirmishes with the guerilla forces of the Aztecs; yet, notwithstanding their constant annoyance and stout resistance, the Spaniards were invariably successful and even managed to secure some booty of trifling value. After a fortnight of rapid marching, fighting and reconnoitering, Cortéz and his men returned to Tezcoco. Here he was met by an embassy from the friendly Chalcans and pressed for a sufficient force to sustain them against the Mexicans, who despatched the warriors of certain neighboring and loyal strongholds to annoy the inhabitants of a town which had exhibited a desire to fraternize with the invading Spaniards. Indeed, the Aztecs saw the importance of maintaining the control of a point which commanded the most important avenue to their capital from the Atlantic coast.